In a typical heap bioleaching process low grade ore containing copper sulphide minerals, usually below 0.5% total copper, is subjected to biological treatment in which agglomerated or un-agglomerated ore is piled onto an impermeable base and then supplied with an efficient leach liquor distribution and collection system.
An acidic leaching solution is percolated through the ore. Microbes growing in the heap produce ferric iron and acid that result in mineral dissolution and mineral solubilization. Aeration in this type of process may be passive, with air being drawn into the heap as a result of the flow of liquid, or active, with air being blown into the heap through piping installed in a lower region of the heap.
A metal-containing leach solution (known as a pregnant liquor solution or PLS) that drains from the heap is collected and subjected to a metal recovery process which typically includes a solvent extraction step. During this step one or more metals contained in the leach solution are transferred into an organic phase of a solvent which has a high affinity for the target metal or metals.
The leaching solution from which the metal has been stripped by the solvent extraction process is referred to as raffinate and is returned to the heap irrigation system, optionally with the addition of acid and nutrients, and is again allowed to percolate through the heap.
For successful heap leaching of sulphide copper minerals microbial activity is required in order to catalyse oxidation of reduced sulphur and iron species. Reference should be made in this regard to FIG. 1 of the accompanying drawings which schematically depicts processes in the bioleaching of primary copper sulphide minerals such as chalcopyrite (CuFeS2) and secondary copper sulphide minerals such as covellite (Cu2S) and chalcocite (CuS). It is to be noted that the bioleaching of primary copper sulphides is normally significantly slower due to the more refractory nature of such minerals. Consequently copper recovery from primary copper sulphide minerals is usually less effective than copper recovery from secondary sulphides for the latter process can often be accomplished in sub-optimal conditions in which microbial catalytic bioleaching activity is inhibited.
In the case of secondary sulphide minerals the microbial oxidation of ferrous iron to ferric iron, at a rate that exceeds the consumption rate of ferrous iron during leaching, without significant sulphur oxidation, is often sufficient to result in significant copper recovery even at ambient temperatures. Ferrous oxidation rates occur rapidly relatively to reduced sulphur oxidation for a number of reasons that include the following: (a) a lower electron yield per mole of ferrous iron than per mole of reduced sulphur; and (b) a greater solubility and mobility of ferrous iron in the ore, compared to corresponding figures for the reduced sulphur species.
The rate of chalcopyrite leaching can be increased if leaching is carried out at an elevated temperature in the range from 40° C. to 65° C. By oxidising reduced sulphur species such as pyrite (FeS2) heat is generated and the temperature of the ore is raised. In order to oxidise reduced sulphur, conditions have to be significantly more favourable and optimised for microbial growth than is the case when only ferrous oxidation is required.
Sub-optimal growth conditions are attributable to at least the following:                1. incorrect pH conditions;        2. a lack of critical macro- and micro-nutrients;        3. a high ionic strength or total salt content of the percolating leaching solution which, as noted, is usually a raffinate solution;        4. the presence of dissolved or entrained organic compounds with inhibitory effects towards microbial growth; and        5. carbon- or oxygen-limiting conditions.        
Total salt content is a measure of the presence of mainly sulphide salts with associated aluminium, magnesium, sodium, calcium and potassium cations or, more generally, any soluble anion or cation, of the percolating leaching solution. When the total salt content is excess of about 80 g/L to 120 g/L microbial activity is inhibited to an increasing extent. Microbial inhibition may however occur at lower levels of total salt content in the presence of particular cations and anions which cause specific inhibition (rather than non-specific ionic strength and osmotic potential inhibition), such as chlorides, nitrates, aluminium, fluoride and arsenic.
In a heap leaching system the target pH of the pregnant liquor solution is typically in the range 1.5 to 2.2. Acid is used principally to dissolve acid-soluble copper and to maintain such copper in solution, and to create an environment conducive to microbial growth and activity. The gangue minerals, however, are often acid-consuming and can react with the acid contained in the solution which is percolated through the heap. This reaction results in the release of salts, typically sulphate salts with associated aluminium, potassium and magnesium cations, that are carried as dissolved species in the solution. The concentration of such dissolved salts increases over time as the heap leaching process progresses and due to the concentrating effect of evaporation.
The increase in organic salts, in the aforementioned manner, results in increasing levels of inhibition of microbial activity. This can be a non-specific inhibition as is caused by high ionic strength (high osmotic potential) which results in reduced water activity which, in turn, results in lowered microbial activity. Alternatively or additionally the inhibition may be caused by specific inorganic compounds such as nitrate, chloride, aluminium, fluoride and arsenic. A common type of microbial inhibition (or sub-optimal microbial activity) encountered in a heap leaching operation is due to high total salt content which results in lowered water activity and non-specific microbial inhibition.
Organic compounds can exhibit a similar inhibitory effect on microbial activity. As has been described the metal which is contained in the pregnant liquor solution is stripped from the solution during a solvent extraction process. Although the solvent is substantially water-insoluble a small fraction of the solvent is indeed soluble and may end up in the water phase. This may be either as water-soluble fractions or as discrete droplets (micelles). The organic compounds, in either form, are then taken up in the raffinate and eventually are percolated through the heap. Some of the organic solvent compounds are inhibitory to bioleaching microorganisms and the introduction thereof into the heap can result in reduced or sub-optimal microbial activity. The organic compounds may be a primary cause of microbial inhibition or may contribute to inhibition effects due to inorganic salts. As the organic compounds are essentially hydrophobic, these compounds will tend to adsorb onto the ore material during migration in the percolating irrigation liquid. Such adsorption effects will have a more detrimental effect on sulphur oxidation than on ferrous iron oxidation. The reason for this phenomena is mainly due to the fact that reduced sulphur compounds are insoluble. Microbial oxidation of such compounds, therefore, have to occur at the mineral surface, and would thus be more negatively affected by surface- adsorbed inhibitory compounds. By comparison, ferrous iron is water-soluble and can readily be oxidized by non-attached microbial cells and is thus less affected by the presence of surface-adsorbed organic compounds.
An elevated temperature is not required for a sulphide heap leaching operation which mainly contains secondary sulphide copper minerals. The oxidation of reduced sulphur, which generates the heat used to elevate the temperature of a heap, is therefore not a strict requirement for the leaching of secondary copper minerals and satisfactory mineral leaching rates can be achieved in the presence of ferrous iron oxidation without significant sulphur oxidation. Since ferrous iron oxidation rates are less affected by sub-optimal microbial conditions than sulphur oxidation conditions the impact of a high salt content or of the presence of organic compounds on the leaching rate of secondary sulphide minerals is relatively unimportant.
Sub-optimal conditions associated with inorganic salts or organic compounds do however have a significant adverse effect on sulphur oxidation which manifests itself in a pronounced way in respect of the mineral dissolution rate of primary sulphide minerals, such as chalcopyrite, where heat generation is a critical factor in achieving a satisfactory leaching rate. Primary copper sulphide and pyrite mineral dissolution rates are negatively affected and the copper leaching rate from primary copper sulphide minerals is reduced.